A study was undertaken to measure the ability of rats to regulate amino acid intake by means of a self-selection method. Weaning rats were simultaneously offered a choice between a 10% casein diet or a 10% amino acid mixture diet lacking methionine or lysine and a 10% casein diet containing different levels of methionine or lysine over 2 weeks. Weight gain and food consumption of rats allowed a choice between the two diets were much greater than those of animals fed on the fixed diets with high levels of methionine or lysine added, and the self-selecting rats had the ability to take in amounts of methionine or lysine appropriate to meet the requirement. Methionine intakes of self-selecting rats were kept to a relatively narrow range (41--100 mg/day), but lysine intakes of self-selecting rats were wide-ranging (86--300 mg/day). It was demonstrated that methionine or lysine intake is closely regulated in rats given a choice of diets containing varied levels of methionine or lysine.